Variable ratio arm bridge



Nov. 8, 1938. M.- J. LOVELADY VARIABLE RATIO ARM BRIDGE Filed Sept. 28, 1935' 2 Sheets-Sheet 2 Patented Nov. 8, 1938 VARIABLE RATIO ARM BRIDGE Maurice J. Lovelady, Gollinnwood, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application September 28, 1935, Serial No. 42,644

2 Claims. (Cl. 175-183) This invention relates broadly to an instrument for measuring electrical properties. More specifically my invention is a direct reading instrument for measuring a relatively wide range the slider S and the junction oi the known and unknown resistances. An alternating current E is applied across the adjustable ratio arms. If the bridge is unbalanced, the alternations of the of values of resistance, capacitance, or inductcurrent will be audible in the telephone receiver. 5 ance. The slider is moved back and forth until a null The art of measuring electrical properties is point is observed in the response of the tele-,

an old one. Perhaps the most widely known inphone receiver.

strument for measuring resistance is the Wheat- I After the balance has been obtained, as indistone bridge; Modifications of the Wheatstone cated by the null point, the unknown value may 10 bridge have been made to adapt it to the measbe determined by multiplying the known or urement of capacitance and inductance. Most standard value by the ratio of the bridge arms instruments of the latter type are cumbersome, A to B. This may be expressed as follows: expensive and complicated. These instruments are mainly used by skilled technicians. I have Unknown xv f standard ,csistance l5 overcome these difliculties by making a light, B

portable, inexpensive bridge which may be used to measure a wide range of electrical constants. It Wm be Observed that the scale for the one of the objects of mymvention is to make slide wire ratio is not linear. That is for small a small light weight portable instrument for movements where A is nearly equal to B the 20 measuring resistance, capacitance, and inducscale Wm be approximately near but as A tance comes much smaller or much larger than B, the

Another object is to make a direct reading inscale Wm become very crowded or Very open- In strument for measuring electrical constants.

an open linear scale is shown on the left Another object is to make a direct reading inand m contrast non'nnear scale the type 25 strument for measuring a very wide range 01 described above is shown on the right. The electrical properties crowded scale is very undesirable and 18 over- A further object is to design a measuring income in the ease of an ordinary Wheetstene strument employing relatively few standards bridge by empleymg a relatively large number which may be selected by simple switching of standards so the ratio arms are generally at 30 means. a balance between the ratios of 4/6 or 6/4.

Additional objects will appear from the ac- In H e Scheme-tie dlagmmendleates One companying specification and appended claims. form of bridge arrangement whleh overcomes My invention may best be understood by the difflculties just described. An adjustable reerence to the accompanying drawings in which sistance M is used for one of the ratio arms. The 35 Figure I is diagram of a conventional other arm is composed of a fixed resistance N wheatstone bridge, whose ohmic value is equal to one tenth the Fig 11 is a schematic diagram of one embodiohmic value of the adjustable arm M at its maxment f my invention, imum setting. A single pole double throw Fig. 111 is a schematic diagram of one of the switch P may be used to connect either the fixed 40 switches used in combination with the circuit resistance M10 arm N or another M10 arm Q of my invention, whose resistance is fixed at an ohmic value equal Fig. IV illustrates a linear and a non-linear t0 the Ohmic resistance of the maximum Setting m, of the adjustable arm M. If the switch P is Fig. V illustrates one form of panel which may connected to the fixed resistance arm Q of the 45 be used, and larger ohmic value, the ratio of M to Q may be Fig. VI is a wiring diagram of my bridge arvaried from .1 to 1. 11? the switch P is connected rangement. to the fixed resistance arm N of the smaller A conventional Wheatstone bridge as repreohmic value the ratio of M to N may be varied sented in Fig. I is comprised of variable ratio from 1 to 10. Thus the use of either of two 50 arms A and B which are serially connected to a fixed arms and one variable arm will not only standard and an unknown resistance or other give a total ratio 0!..1 to 10 or 1 to but also electrical unit whose resistance is to be deterthe ratio may be read directly on a linear scale, mined. An indicating device D, such as a teleas shown in Figure V. Although the arrangephone receiver or the like, is connected between mentjust described may be used in the measure- 55 ment of resistance, modifications are required in the measurement .of inductance or capacitance.

The reactance of a capacity, assuming a constant frequency is impressed, will vary inversely with the size of the capacity. That is, at a fixed frequency, the larger 9. capacity the smaller will be its reactance. If the straight direct reading bridge arrangement is employed, as just described, the balance might be made to a null point but the scale reading would be erroneous. The true reading would be the reciprocal of the scale reading. The difiiculty may be overcome by reversing the relative position of the known and unknown capacitance. One simple method is to use a double pole double throw switch T as reversing means as shown in Figure 111. a 1

A capacitor or an inductor ordinarily does not offer pure reactance but has as well a finite resistance. The resistance of a capacity or inductor ordinarily broadens the balance or null point. If the resistance of the standard capacitor or standard inductor is balanced against the resistance of the unknown capacitor or unknown inductor, the effect of the two resistances will be eliminated and the capacitance or inductance may be determined by exactly balancing the bridge. Since the resistance of the standard may be greater, equal to, or less than the unknown, it is necessary to put a variable phase balancing resistor U in series with either the known or unknown. This may be accomplished with a single pole double throw switch V, as shown in Figure III. The complete wiring diagram also shows means for short circuiting the phasing resistor when the bridge is arranged for resistance measurements.

The bridge circuits described above are schematic in form. A practical alternating current bridge requires an amplifier to bring up the output to a point where telephone receivers may be used. It is customary to'employ audio currents of a frequency of one thousand cycles;

therefore to be complete the bridge should inelude an amplifier and a source of one thousand cycle alternating current. The complete ,circuit arrangement is shown in Figure V-I. 7

A source of alternating current, such as 110 volts 60 cycles, is connected to the input I, 3 of the rectifier system which is included-within;- the broken lines 5. The rectifier is of the well.

. to a common chassis terminal and not to. actual the grid circuit.

earth. An actual earth connection might short circuit the alternating current input line.

Within the broken line IS a thermionic tube I1 is connected to generate oscillations of a frequency of 1000 cycles; although other frequencies may be used. The grid I9 is connected through a grid leak grid condenser combination 21 to a circuit 23 tuned to 1000 cycles. The anode 25 is coupled'through the transformer 21 to The resistor 29 reduces the anode voltage supplied by the rectifier to an appropriate value. The resistor 29 may be by passed by a capacitor 3I. A tertiary winding 33 on the transformer 21 is connected to the transformer supplies the alternating current for the bridge, which will be described below.

The output of the bridge is represented by a ground connection I and a capacitor 43 which connects the output of the bridge to the input of a two stage audio frequency amplifier. The

amplifier is represented by the double purpose thermionic tube 45 and the connections within the broken lines 41. The grid, 49 which connects to the coupling capacitor 03, is returned to ground through a grid resistor 5|. The anode 03 is connected through an iron core choke '55 to the positive terminal of the power supply. The grid 51 of the second amplifier is coupled to the iron core choke by a capacitor 59. The grid is returned to ground through a grid resistor GI. The output of this amplifier comprises its anode 63, the primary 65 of transformer 61, a resistor fourth, fifth and sixth contacts 61 the multi-position switches 3|, 33. The three capacitance standards 95, 91, 99 are connected to the seventh, eighth and ninth contacts of the multi-position switches. The movable contact arms IOI, I03 of these switches are moved with a single control I05, as are the movable arms I01, I09 of the double throw reversing switch III. The double throw switch is'used to reverse the standard and unknown connections for capacitance measurements as previously described. Two of the fixed contacts H3, N5 of the reversing switch I II are connected to one of the terminals II1 to which the element of uknown value is to be connected. The other of the terminals H1, is connected to the variable phasing resistor H9 and to the fixed contact I2I of the single pole double throw switch I23. The second variable phasing resistor I25, is a vemier, which is used for fine adjustment and is serially connected to the first variable resistor I I9. The remaining fixed contact I21 of the single pole double throw switch is connected to the lower terminal of the Vernier phasing resistor I25 and the movable contact arm IOI of the multi-position switch 8|. The movable contact I29 of the single pole double throw switch is connected to ground.

The ratio arms of the bridge are-composed of either of two fixed resistors I3l, I33, which are similar to N and Q in Figures If and III. These resistors are connected together and to the slider I35 of the variable resistor arm I31. The lunction of the fixed resistors and the slider is connected to the coupling capacitor 43 which is connected to the input of the audio amplifier. The voltage input to the bridge is represented by the secondary 39 of the balanced transformer 31. The secondary is connected to the movable contact I20 of the single pole double throw switch I30 and to the fixed terminal of the variable resistor I31. The secondary is also connected to the movable contact arms I01, I09 of the double throw double pole switch III. The remaining fixed contacts of the double pole double throw "switch are connected together. The

movable contact I03 of the lower multi-position switch 83 is connected to the fixed contact I39 of the double pole double throw switch.

It should be noted that the movable contacts of the double throw reversing switch and the movable contacts of the multi-position switches are operated with a uni-control. To avoid difliculties the double throw reversing switch has contact arrangements which automatically operate when the switch is positioned for capacitance readings. A second switch 5 is operated in conjunction with the single pole double throw ratio arm switch I30. This switch I connects a small variable capacitor between the high potential terminal of thehlghest valued standard resistor and ground. The small variable capacitor is only used when resistances of very high value are being measured. A high resistance in the unknown arm may have a substantial capacity reactance to ground which would unduly broaden the null point. This may be balanced out by adjusting the small variable capacitor. By the way of example, I shall give a table of values of the several elements of the bridge. It should be understood that other constants may be used, but I have found the following useful over a vary wid range of values:

Standards of inductance 1 henry 10 millihenries 1000 microhenries 1 microfarad .01 microfarad .0001 microfarad Standards of capacitance"- farads Partly inherent capacity.

With the above standards and variable ratios, resistances from 1 to 1,000,000 ohms, inductances from 100 microhenries to 10 henries, and capacities from 1 micromicrofarad to 10 microfarads may be measured and their values determined directly from the variable resistor arm scale.

The entire device may be assembled within a small portable case 9% inches long, 6 inches high, and 4 inches deep. The total weight is about 5 /2 pounds. The front panel of the case is shown in Figure V. The adjustable ratio arm pointer 2M with its linear scale 203 is shown in the central portion of the illustration. The control knob 205 connects to the nine position switch. The double throw switch 201 in the upper right,hand corner is the ratio selector switch. The small knob in the lower right is the control for the variable capacitor which is used to balance out the capacity reactance of high resistances. To the left of this small control knob and in the following order are the off-on switch 209, the phone jack 2| I, the double throw switch M3 for placing the phasing resistors in the standard or unknown arms, and the knob 2|5 for the vernier phasing resistor. Immediately above the vernier phasing adjustment knob, is the control knob 2i! for the coarse phasing adjustment. In the upper left hand corner of the panel are two binding posts which are the terminals 2|! for the unit of unknown value. The power supply is represented by a cord and plug 22!.

The method of using the bridge is as follows: An unknown resistance is connected across the unknown terminals, the ratio selector switch is placed on Low, the nine position selector switch is placedon the lowest value of resistance (1-10), and the variable ratio arm is moved slowly from 1-10. The response in the telephone receivers will be a 1000 cycle note which will disappear or become a null at the balance point and will increase in intensity on either side of the balance point. If no balance is found on this scale the ratio selector switch is turned to high and the variable ratio arm adjusted for a balance. If a balance is found on this range the value of the unknown resistance will be from 10 to 100 ohms and will be ten times the scale reading at the balance point. If the unknown resistor has an ohmic value greater than 100, the above described operation is repeated on the higher resistance standards until a balance is found.

The measurement of inductance is performed in substantially the same manner. The only difference is that the resistance component of the inductor must be balanced out by adjusting the phasing resistors. The usual method is to move the variable resistor arm to an approximate balance position and then move the coarse phasing adjustment to balance out the residual 1000 cycle note. The final adjustment results from successive adjustments of the vernier phase control and the variable ratiov arm. At the exact balance point the residual 1000 cycle note will disappear and a true zero response will be found.

Capacity is measured in accordance with the procedure for measuring inductance. However, one additional step is required. There is a small amount of inherent capacity. in the terminals and leads. This value is usually 25 to 30 micromicrofarads. The unknown terminals are left open and the bridge is carefully balanced. The reading for the balance is the inherent capacity which must be subtracted from the final reading which is obtained after the unknown capacitor has been connected and balanced.

I have described a compact, self-contained portable bridge. Its accuracy will be determined largely by the precision of its standards and the care with which the variable ratio arms are chosen. Accuracy within about 2% is well within practical design. The bridge lends itself to ready measurements within a wide range of values of resistance, inductance or capacitance. The foregoing description, constants and circuits are merely by way of example. Those skilled in the art will find numerous modifications within the scope of my invention which is only limited by the prior art and the appended claims.

I claim as my invention:-

1. In a portable measuring device of the Wheatstone bridge type, the combination which includes a continuously variable resistor operable in cooperation with a linear scale, a first and a second fixed resistance, a first switch for selectively connecting one of said fixed resistors to said variable resistor whereby said variable and said selectable fixed resistors form the conjugate variable arm and ratio arm of said bridge respectively; a plurality of standards of known values of resistance, capacitance, and inductance, a second switch to select one of said standards, terminal connections for an unknown capacitor, resistor, or inductor which is to be measured, connections including 8- 4- I amass? reversing switch wherebysaid unknown and said I a tandard are serially connected across said variable arm and said ratio arm to form the remain- 'ing' twoconjugate arms of said bridge, said reversing switch being operably connected to saidsecond switch whereby the selection of a capacitive standard automatically reverses the relative position of said standard and said unknown; 9. second variable resistor, and a third switch whereby said second variable, resistorvmay be and connections whereby said fourth switch con-- nects said variable capacitor across at least one of said resistance standards to thereby. permit the inherent shunt capacitance across high r'e-' sistances which are to be measured to be balanced out; means for energizing said bridge, and means for indicating a balance.

- 2. A device of the character described in claim 1 which is further characterized in that said means for energizing said bridge is an audio ire-'- quency vacuum tube oscillator. MAURICEJ. LOVELADY. 

